1. Field of the Invention
This invention relates generally to photonic integrated circuits (PICs) and more particularly to multiplexers (MUXes) and demultiplexers (DEMUXes) employed in photonic integrated circuits (PICs). The devices disclosed here are more particularly for use in optical transmitter photonic integrated circuits (TxPICs) and optical receiver photonic integrated circuits (RxPICs) having an integrated on-chip optical combiner of the wavelength selective type, in particular, an elliptical supergrating MUX or DEMUX or a Echelle grating MUX or DEMUX, in lieu of an arrayed waveguide grating (AWG) MUX or DEMUX.
2. Description of the Related Art
The employment of monolithic photonic integrated circuits (PICs), also sometimes referred to as planar lightwave circuits (PLCs), are on the rise in deployment in optical telecommunication systems. These devices provide the integration of both active and passive optical components on a single substrate and are integrated with other optical components to form a multi-functional optical device for use in such systems. The gravitation to PICs is strong because it leads to utility of providing an entire system function, let alone a component function, in a single chip in a single package. Compared to the deployment of discrete optical components, such monolithic PIC chips can significantly reduce the size of optical components necessary in the optical system, albeit in a transmitter photonic integrated circuit (TxPIC) or a receiver photonic integrated circuit (RxPIC), for example, as well as significantly reduce the overall costs in a system. Examples of recent advanced TxPICs and RxPICs are disclosed in U.S. patent applications, Ser. Nos. 10/267,331; 10/267,304; 10/267,330; and 10/267,346, supra.
The size of a TxPIC InP-based chip having ten signal channels that comprise an array of ten laser sources, such as, for example, DFB lasers, and an array of corresponding electro-optic modulators, such as, for example, electro-absorption modulators (EAMs), and a wavelength selective combiner in the form of an arrayed waveguide grating (AWG) is about 4 mm by 4.5 mm. It would be desirable to reduce the size of such chips while simplifying the combiner structure and reducing its on-chip insertion losses.
One such candidate for an on-chip combiner is the elliptical supergrating MUX and DEMUX. An example of this type of device is disclosed in the article of Yankov entitled, “Multiwavelength Bragg Gratings and Their Application to Optical MUX/DEMUX Devices, IEEE Photonics Technology Letters, Vol. 15(3), pp. 410-412, March 2003 as well as in U.S. patent application publication No. 2003/0210862, published Nov. 13, 2003. Also, in particular the elliptical supergrating DEMUX is illustrated in U.S. patent application publication No. 2004/0036933, published Feb. 26, 2004 which discloses a so-called planar holographic multiplexer/demultiplexer comprising a series of curved or elliptical gratings that reflect different wavelengths and combines them at a predetermine output or input position. These gratings are referred to as holograms in the context that sets of such gratings being independently wavelength selective for a particular peak wavelength among other wavelengths in a multiplexed output and provide refractive index modulation and reflection of a selected peak wavelength This publication also cites the previous work of Henry et al. entitled, “Four-Channel Wavelength Division Multiplexers and Bandpass Filters Based on Elliptical Bragg Reflectors”, Journal of Lightwave Technology, Vol. 8(5), pp. 748-755, March, 2003 and in U.S. Pat. No. 4,923,271 which discloses a series of Bragg reflectors for reflecting multiple wavelengths from or to a central point of the combiner or decombiner. It is pointed out in publication No. 2004/0036933 that the Henry device has the disadvantage of not being scalable to high channel count because, since the gratings are spatially separated and will increase significantly with the number of added channels so that the device size becomes unyielding as well as its functionality significantly deteriorates. Publication '933 also mentions that in such two-dimensional devices, there may be the problem of intersection in the grating reflection field of the intersection of gratings of the supergrating to cross one another at intersections since the groups of subgratings are designed for different wavelengths. Moreover, if dashed lines are employed for the gratings, the spacing between dashes may be varied so that the reflection coefficient is enhanced and potential destructive interference (crosstalk) between intersecting gratings of different sets can be minimized. In other words, this destructive interference can be reduce to some extent by diminishing the overlap of the holograms or sets and this can be carried out by using dashed or dotted line gratings in the different sets. However, it is unclear how that might be successfully accomplished.
Another candidate is the Echelle grating for which much work has been published including, as examples, U.S. Pat. Nos. 5,206,920; 6,339,662; 6,141,152; and U.S. published patent application, Publication No. 2002/0081061, as well as the article of Janz et al. entitled, Planar Waveguide Echelle Gratings in Silica-On-Silicon”, IEEE Photonics Technology Letters, Vol. 16(2). pp. 503-505, February, 2004.
While some of these publications indicate that these candidates can be used in photonic integrated circuits (PICs), such as in Publication No. '933 at page 6, paragraph, there is no indication or teaching as to how this might be affected or accomplished.